1. Field of the Invention
A method and an apparatus for detecting defects in the composite materials of a wind turbine blade or similar type of structure using an infrared (IR) camera, where the defects may be separation of the layers between different composite materials, waves or wrinkles in the composite layers, separation of the layers within the composite material, layers in the composite where the resin did not fully wet the cloth layer, or liquid such as water that may have infiltrated such layer defects. The temperature of the wind turbine blade relative to the ambient temperature is changed in such a way as to produce IR intensity changes in the region of the defect that can be visually detected, or detected using a computer and a signal processing algorithm. The same approach will work on other composite structures such as those found on aircraft.
2. Brief Description of Prior Art
The method and apparatus of the present invention is motivated by the need for a low-cost, reliable, and fast method of identifying defects in the composite materials of a wind turbine blade, or other similar type of composite structure, where the defects may be separation of the layers between different composite materials, waves or wrinkles in the composite layers, separation of the layers within the composite material, layers in the composite where the resin did not fully wet the cloth layer, or liquid such as water that may have infiltrated such layer defects. While IR methods have been used to detect defects such as these in wind turbine blades, these methods have generally been limited to defects near the surface. In addition, for some of the methods, the time required to complete a measurement has been too long to be operationally attractive. The composites comprising these blades might be 20 to 40 mm thick, or more.
FIG. 1a shows a typical cross-section of a wind turbine blade 10, and FIG. 1b illustrates two types of defects, delamination 38 and lack of glue 40, that could be present in a wind blade 30. FIG. 1a shows the leading edge 12 of the wind blade, the upper shell 14, the lower shell 18, the trailing edge 16, the main spar 20, and the inside of the blade 22. In FIG. 1b, the wind blade shell 30 is comprised of a glass fiber reinforced plastic (GFRP) skin layer 32, a glue layer 34, and a thicker GFRP wall 36. Defects from lack of glue 40 and delamination 38 are illustrated. The preferred embodiment of the present invention requires fluid access to the inside of the blade, and will work on any structure that allows a fluid to be transported to all sections of the inside of the composite. The alternative embodiment only requires access to one side (i.e., the outer side) of the composite.
FIG. 2 shows a typical cross-section of a wind turbine blade 50 based on the blade 10, 30 shown in FIGS. 1a and 1b and illustrates potential locations of defects that we evaluated the performance of the present invention. It was assumed that the composite was 20-mm-thick 58 and was comprised of three layers 52, 54, 56 with the possibility of defects 60, 62, 64, 66, 68 occurring in each layer 60, 66, 68 and between the layers 62, 64. The composite is comprised of three sections: (1) a 3-mm GFRP layer (glass fiber reinforced plastic) on the top of the blade 52, (2) a 12-mm GFRP layer on the bottom of the blade 56, and (3) a 5-mm glue-form layer separating these two layers 54.
Previously, we have developed an IR inspection method for aircraft honeycomb structures using a conductive heating method that can be used to detect defects in wind blade composites. This method entailed heating the surface of the composite conductively with a silicon heating mat for a short period of time (10 s) and then analyzing the IR image for defect obtained using an uncooled IR camera. Each measurement had a coverage area of approximately 2 ft2. This method, however, will have difficulties in detecting the presence of defects 68 in the deepest portion of the composite structure, particularly in the deeper portions of the third layer 68. Since the method takes 30 s or more to complete a measurement, with each measurement covering only several square feet of surface area, it could take the better part of a day to completely inspect a wind turbine blade.
FIG. 3 illustrates the temperature of the defect signal 70 occurring from a defect 64 found between the glue-form layer 54 and the bottom GFRP layer 56 using the conductive method of inspection. In this case, the composite structure is heated conductively for about 10 s to increase the temperature of the surface of the composite and then removed. Only small temperature changes are required (e.g., typically 3° C.) to produce a detectable IR signal 70 that is significantly greater than the background IR intensities. For this computation, we assumed a 1-in. by 2-in. delamination-type defect and a 6° C. temperature change. The model result, which has been validated by actual IR measurements, clearly indicates that the conductive method of heating will easily detect such a defect. The model results also suggest that the method will have problems detecting defects closer to the inside surface of the composite deep within the third layer of the composite.